The TMC2100 has an internal clock but it can be used with an external user provided clock as well. If the internal clock is used, the CLK pin must be tied to Ground. It must never be left floating. If an external clock is used, the first rising edge on the CLK pin switches to external clock. It must be made sure that the external clock is always present during operation. If it is missing, the driver should be disabled using ENN pin to prevent the driver stage from remaining in continuous on conditions.